OOOPS!
Green Granite,
Oh Dear, yes, how embarrassing! Should have been 90mph!!! I have left the original post so that folk understand what you meant!
Now, as to WHY the aquaplaning speed is dependent on tyre pressure. I believe that a lower-pressure tyre, when confronted with a wedge of water in front of it, will deform more readily, allowing the wedge to further separate the tyre from the surface.
Once the tyre leaves the surface, it will tend to stop rotating, but I've never seen a formula to suggest any kind of modelling of what happens in this regime.
TheOddOne (mathematically challenged this morning!)